Yes, Cakewalk. Or, I guess they're calling their consumer music creation software Music Creator now. I don't know about a demo. I'm thinking probably not. I browsed a bit at SourceForge and didn't see anything, but you could give that a try maybe.

Suggests to try FlexiMusic Composer, a multi-track program for composing, editing and mixing of music using sampled instrument sounds. Software is easy to use, downloadable and free to use as trail version. http://www.fleximusic.com/composer/overview.htm

Are you going to be using a keyboard? (electric piano type, not the typing one). If so, I will point you in the direction of Midi. I'm 98% certain you can also do this with just about any other instrument as well.

Basically, you download from the THOUSANDS upon THOUSANDS of already Midi sequenced instruments; This allows you to make an instrument sound like a drum, a flute or even an elephant (if you so desire)... Or, like those accursed Jingle cats we are tortured into hearing every Christmas.

Enjoy, the info. should point you in the right direction. (remember, google is your friend)

Marama, I am a musician, I play the piano and organ. I have a Yamaha PSR 520. After rereading what you asked for, I am confused as to what you want to accomplish with your PC, musically, with a 'Cakewalk' like, software. Do you mean that when you press the keys of your PC keyboard, sounds of a preprogrammed instrument will come out of the speakers at a pre- denoted note, such as C#, one octave above middle C? Or, do you want to transfer pre written music from the PC, to the Midi source(piano or synth), for playing there and/or overdubbing(adding a singing voice and/or instrumental voice/s?
I did this when I was still in the music business on stage, I would have prerecorded music to play during the band breaks. It also works for recording sessions. I had my parts all prerecorded so I could press a key to start the prerecord, which would kick in at exactly the right note and I could walk away from the keyboard and the keyboard would continue to play the song the band was doing, as if I were there still pressing the keys.

Marama, I think I've got just the program for you. It's at www.jamstudio.com. It is literally the easiest music making program I've ever used. All you do is click on the chords you want played, pick your instruments and press play. They've got hundreds and hundreds of instrument variations. I made the mistake of buying Cuebase and only found myself pulling my hair out. Too, too hard. The other thing is that most of JamStudio is free.